Acrylonitrile polymer is manufactured by the producer in the form of filaments and converted to staple fibers by the producer or by his customer in the trade. Fibers and filaments of acrylonitrile polymer, i.e., acrylic fibers, are relatively difficult to dye. Although the producer usually adapts the acrylic fiber for improved dyeability by incorporation of copolymeric dye sites and other techniques, the dyeing step is usually carried out in the trade by using carefully controlled conditions and lengthy dyeing times.
The economic and technical advantage of dyeing as-spun acrylic filaments in the process of their manufacture is apparent, and methods to accomplish this have been developed for the wet-spinning process of producing acrylic filaments. These methods are described in, e.g., Cresswell U.S. Pat. No. 2,558,735, Moore U.S. Pat. No. 3,113,827, Wirth et al. U.S. Pat. No. 3,111,357, Briar et al. U.S. Pat. No. 3,296,341, and Japanese Pat. No. 44-24495 (1969). However, because of the inherent differences between wet- and dry-spinning procedures, the techniques that have been developed for dyeing wet-spun acrylic filaments are not easily adaptable for use with dry-spun acrylic filaments. For example, wet-spinning dyeing procedures generally require the formation of a gel of the filament which, of course, is not feasible in a dry-spinning operation. Consequently, methods have been sought to dye dry-spun acrylic filaments on the run during the production of the filaments. This has been termed the "producer dyeing" of filaments.
In the manufacture of producer colored acrylic filaments, it is desired in actual commercial practice to introduce the dye into the fiber at some stage after the spinning step. Otherwise, to make a desired change in the product color would necessitate interrupting the spinning process and flushing of the color from the system during the transition from one color to another. The acrylic filaments are therefore usually collected at the spinning step, and prior to the next step the spun filaments are usually stored for several hours or more.
In accordance with the present invention, it has been found that it is essential to maintain the dry-spun acrylic filaments at a relatively low level of water content after they have been spun but prior to the application of dye to the spun filaments. More specifically, it has been found that the water content of the spun filaments should not exceed 30%. It has also been found that it is critical to draw the filaments while they are still in contact with the liquid dye mixture. If the filaments become soaked with water before they are dyed, or if they are not drawn while still in contact with the liquid dye mixture, only a relatively low level of dye is absorbed into the fiber.